You do not need to fill the bin to the top. You can start the ferment cycle whenever you like.
Green mold is a sign that there is rotting happening, instead of fermentation. With fermentation, you will sometimes see white mold, which is fine and good, but not mold with any color to it. If you see colored mold, like green, then either you haven’t added enough bokashi bran to the foods throughout the bin, or perhaps your bokashi bran is not viable anymore. Bokashi bran will keep for well over a year if its stored near room temperatures, and out of direct sunlight. Also, be sure that you dont add any foods to the bin that are already rotting, as this rotting can spread.